The present invention relates to the apparatus and method of network synchronization in an asymmetric two-way satellite communication system. More particularly, the present invention relates to an apparatus and method of network synchronization that broadcasts network synchronization information to all remote stations using adaptation field structure that is defined at the MPEG-2 system standard. In addition, all remote stations maintain synchronization between the central station and remote stations using network synchronization information that is located at a program clock reference part and an original program clock reference part of the adaptation field structure.
In conventional satellite communication system, there are several well-known methods for network synchronization. One method is to broadcast a pilot signal from the central station to remote stations. A second method is to broadcast a synchronous word periodically. A third method is closed loop synchronous method. Another method is to use frame timing of a forward time division multiplexing (TDM) stream.
In the first method, a pilot signal is generated at the central station with the help of high-accuracy reference frequency generator at the central station, and the pilot signal is broadcast through dedicated channels. All remote stations receive the pilot signal and establish clock and frequency synchronization.
In the second method, in every frame that is composed of a number of slots, the central station broadcasts synchronous words to remote stations
In the third method, every remote station transmits locally-generated transmission data symbol timing to the central station. The central station measures the error between the transmission data symbol timing and the timing generated by the standard symbol timing generator and sends the error to remote stations through forward data stream. Remote stations compensate the received error and establish timing synchronization between them and the central station.
In the final method, every remote station extracts frame timing from the received TDM stream and uses them as a transmission slot timing of burst data.
The first method is appropriate for continuous mode synchronous communications. But it deteriorates the efficiency of the frequency spectrum. The second method is problematic unless the length of a TDM frame is a multiple of the length of a slot. In addition, a system in which a number of data streams are remultiplexed and thereby the location of a synchronous word is modified is not compatible with the second method. Likewise, the final method is not compatible with a system in which a number of data streams are remultiplexed and thereby the location of a synchronous word is modified. The conventional methods are not appropriate for an asymmetrical multimedia satellite communication system such that the forward stream is a TDM stream including MPEG-2 transmission packets, the length of transmission burst at remote stations is not a multiple of the length of the MPEG-2 transmission packet, or a number of data streams are remultiplexed.
To overcome the foregoing inconveniences, the present invention generates MPEG-2 transmission packets for network synchronization using adaptation field structure and broadcasts them to remote stations. Then, the network synchronization reception part of the central station and the remote stations extract network synchronization information from the adaptation field of the broadcast network synchronization packet and obtain a reference timing. Using the reference timing, the apparatus of network synchronization in accordance with the present invention generates transmission-reception slot timings for synchronous burst transmission. If a number of data streams are remultiplexed and thereby the location of a synchronous word is modified, the error between the transmission data symbol timing and the timing generated is manipulated by the standard symbol timing generator, and the original location of the network synchronization packet is determined in order to generate transmit-receive slot timing.
The present invention to provides an apparatus and method of network synchronization in an asymmetric two-way satellite communication system, which generates MPEG-2 transmission packets for network synchronization using adaptation field structure and broadcasts them to remote stations. All remote stations maintain synchronization between the central station and the other remote stations using network synchronization information that is located at a program clock reference part and an original program clock reference part in the adaptation field structure.
An apparatus of network synchronization in asymmetric two-way satellite communication system in accordance with the present invention comprises a central station and a plurality of remote stations. A transmission part of the central station comprises network synchronization information generating means, transmission stream generation means, and transmission means. The transmission stream generation means receive the network synchronization information and generate a network synchronization packet containing the network synchronization information in adaptation field structure. The transmission means multiplex, modulate and broadcast the network synchronization packet and a transmission stream.
The reception part of the central station comprises reception processing means, reception slot timing generation means, and synchronous burst reception means. The reception processing means extract network synchronization information from the received transmission stream and generate a reference timing signal. The reception slot timing generation means receive the reference timing signal and generate a reception slot timing. The synchronous burst reception means receive the reception slot timing, measures reception timing error, and supplies the reception timing error to the transmission stream generation means.
The remote stations comprise reception processing means, transmission slot timing generating means, and synchronous burst transmitting means. The reception processing means extract network synchronization information from the received transmission stream and generate a reference timing signal. The transmission slot timing generating means generate transmission slot timing on the basis of the reference timing signal and adjusts transmission slot timing generation time on the basis of the reception timing error received from the central station. The synchronous bursts transmitting means transmit burst in synchronous with the transmission slot timing.
A method of network synchronization in an asymmetric two-way satellite communication system in accordance with the present invention basically comprises the following five steps. The first step is to make a network synchronization packet by including network synchronization information in an adaptation field structure and multiplex and then broadcast the network synchronization packet. The second step is to extract network synchronization information from the received network synchronization packet and generate a reference timing signal on the basis of the network synchronization information. The third step is to generate a reception slot timing signal at the reception part of the central station on the basis of the reference timing signal and generate transmission slot timing signal at the remote stations. The fourth step is to measure the timing error between the reception slot timing signal and the slot timing signal of the received data and transmit the measured timing error to the remote station when the remote station sends out data in synchronization with the transmission slot timing signal. The fifth step is to adjust the transmission slot timing generation time at the remote station receiving the measured timing error and to generate a transmission slot timing signal.
More particularly, in the method of network synchronization in accordance with the present, the program clock reference (PCR) area and the original program clock reference (OPCR) is replaced by the network synchronization information in the adaptation field structure. Using remultiplexers to adjust the program clock reference, the remote stations find the original location of the network synchronization packet by the difference between the program clock reference (PCR) and original program clock reference (OPCR) and the reference timing to generate burst transmission timing.
In other words, using the adaptation field that is a component of the MPEG-2 transmission stream in an asymmetric two-way satellite communication system, network synchronization packets are broadcast to all remote stations. Network synchronization information has been replaced with program clock reference (PCR) and original program clock reference (OPCR) with the help of the adaptation field that is part of the forward MPEG-2 transmission stream in the system. Each remote station obtains reference timing by processing received network synchronization packets of MPEG-2 transmission stream and generates transmission slot timing for synchronous burst transmission. After obtaining reference timing, the central station measures the error between reception slot timing and reception timing of the burst transmission and sends the error to the remote station with the help of the forward MPEG-2 transmission stream. The remote station generates transmission slot timing that is adjusted with the error.